Inclusion body myositis (IBM), is the most common acquired disease of muscle in adults over the age of 50. IBM has a typical clinical presentation but requires muscle biopsy for definitive diagnosis. Although, there is compelling evidence for the importance of immunologic abnormalities in it pathogenesis, the cause of the disease is not known and it is generally considered to be resistant to treatment with conventional immunosuppressive agents. Beta interferon-1a (betaINF1a) (AVONEX), an immunomodulatory cytokine, is a candidate therapeutic agent for IBM. The proposed pilot study is a six-center, randomized, placebo-controlled, parallel group study of 30 mcg of betaINF1a, administered intramuscularly once a week in 30 patients with IBM. The primary goals of the study are to examine the safety and tolerability of betaINF1a in patients with IBM, refine outcome measures of muscle function in IBM, and extend previous data on the natural history of IBM. A secondary goal is to obtain preliminary data on the efficacy of betaINF1a by measuring changes in muscle strength and mass. The study will include only patients that meet rigid criteria for IBM and who do not have complicating illness. The testing methods have been validated by the investigators in other studies of muscle disease. The information obtained in this study will be essential for the effective design and conduct of pivotal trials in IBM.